U.S. patent application number 14/762940 was filed with the patent office on 2015-12-10 for cigs film production method, and cigs solar cell production method using the cigs film production method.
This patent application is currently assigned to NITTO DENKO CORPORATION. The applicant listed for this patent is NITTO DENKO CORPORATION. Invention is credited to Jakapan Chantana, Kazunori Kawamura, Takashi Minemoto, Hiroto Nishii, Seiki Teraji, Taichi Watanabe, Yusuke Yamamoto.
Application Number | 20150357492 14/762940 |
Document ID | / |
Family ID | 51353909 |
Filed Date | 2015-12-10 |
United States Patent
Application |
20150357492 |
Kind Code |
A1 |
Teraji; Seiki ; et
al. |
December 10, 2015 |
CIGS FILM PRODUCTION METHOD, AND CIGS SOLAR CELL PRODUCTION METHOD
USING THE CIGS FILM PRODUCTION METHOD
Abstract
A CIGS film production method capable of suppressing oxidation
of a front surface of a CIGS film, and a CIGS solar cell production
method using the CIGS film production method includes the steps of:
forming a first region having a Ga/(In+Ga) ratio progressively
reduced as the thickness of the first region increases to a
predetermined first thickness position from a back surface of the
CIGS film; forming a second region having a Ga/(In+Ga) ratio
progressively increased as the thickness of the second region
increases to a predetermined second thickness position from the
first region; and forming a third region on the second region by
vapor-depositing Se and In, the third region having a Ga/(In+Ga)
ratio progressively reduced toward a front surface of the CIGS
film.
Inventors: |
Teraji; Seiki; (Ibaraki-shi,
JP) ; Watanabe; Taichi; (Ibaraki-shi, JP) ;
Nishii; Hiroto; (Ibaraki-shi, JP) ; Yamamoto;
Yusuke; (Ibaraki-shi, JP) ; Kawamura; Kazunori;
(Ibaraki-shi, JP) ; Minemoto; Takashi;
(Kusatsu-shi, JP) ; Chantana; Jakapan;
(Kusatsu-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NITTO DENKO CORPORATION |
Ibaraki-shi, Osaka |
|
JP |
|
|
Assignee: |
NITTO DENKO CORPORATION
Ibaraki-shi, Osaka
JP
|
Family ID: |
51353909 |
Appl. No.: |
14/762940 |
Filed: |
January 24, 2014 |
PCT Filed: |
January 24, 2014 |
PCT NO: |
PCT/JP2014/051506 |
371 Date: |
July 23, 2015 |
Current U.S.
Class: |
438/95 |
Current CPC
Class: |
H01L 31/0749 20130101;
H01L 31/0322 20130101; H01L 31/1884 20130101; H01L 31/03923
20130101; H01L 31/18 20130101; Y02P 70/521 20151101; H01L 21/02568
20130101; H01L 21/02485 20130101; Y02P 70/50 20151101; Y02E 10/541
20130101; H01L 21/0251 20130101 |
International
Class: |
H01L 31/0392 20060101
H01L031/0392; H01L 31/18 20060101 H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 12, 2013 |
JP |
2013-024575 |
Claims
1. A production method for a CIGS film to be used as a light
absorbing layer for a CIGS solar cell, the method comprising the
steps of: forming a first region having a Ga/(In+Ga) ratio
progressively reduced from a back surface of the CIGS film toward a
front surface of the CIGS film; forming a second region on the
first region, the second region having a Ga/(In+Ga) ratio
progressively increased toward a front surface of the CIGS film;
and forming a third region on the second region by vapor-depositing
selenium (Se) and indium (In), the third region having a Ga/(In+Ga)
ratio progressively reduced toward the front surface of the CIGS
film; wherein the Ga/(In+Ga) ratios are each defined as a ratio of
a gallium (Ga) atomic number concentration to a sum of an indium
(In) atomic number concentration and the gallium (Ga) atomic number
concentration.
2. A CIGS solar cell production method comprising the step of:
forming a rear electrode, a light absorbing layer, a buffer layer
and a transparent electrically-conductive film in this order on a
substrate; wherein the light absorbing layer is a CIGS film formed
by the CIGS film production method according to claim 1; wherein
the CIGS film has a back surface located adjacent to the rear
electrode.
3. A production method for a CIGS film to be used as a light
absorbing layer for a CIGS solar cell, the method comprising the
steps of: forming a first region having a Ga/(In+Ga) ratio
progressively reduced from a back surface of the CIGS film to a
predetermined thickness position of the CIGS film, and a second
region having a Ga/(In+Ga) ratio progressively increased from the
predetermined thickness position toward a front surface of the CIGS
film; and forming a third region on the second region by
vapor-depositing selenium (Se) and indium (In), the third region
having a Ga/(In+Ga) ratio progressively reduced toward the front
surface of the CIGS film; wherein the Ga/(In+Ga) ratios are each
defined as a ratio of a gallium (Ga) atomic number concentration to
a sum of an indium (In) atomic number concentration and the gallium
(Ga) atomic number concentration.
4. A CIGS solar cell production method comprising the step of:
forming a rear electrode, a light absorbing layer, a buffer layer
and a transparent electrically-conductive film in this order on a
substrate; wherein the light absorbing layer is a CIGS film formed
by the CIGS film production method according to claim 3; wherein
the CIGS film has a back surface located adjacent to the rear
electrode.
Description
TECHNICAL FIELD
[0001] The present invention relates to a CIGS film production
method for producing a CIGS film having a Ga/(In+Ga) ratio varying
along its thickness, and a CIGS solar cell production method using
the CIGS film production method.
BACKGROUND ART
[0002] Thin film solar cells typified by amorphous silicon solar
cells and compound thin film solar cells allow for significant
reduction in material costs and production costs as compared with
conventional crystalline silicon solar cells. In recent years,
therefore, research and development have been rapidly conducted on
these thin film solar cells. Among these thin film solar cells, a
CIGS solar cell which is a type of compound thin film solar cell
produced by employing Group I, III and VI elements as constituents
and including a CIGS film composed of an alloy of copper (Cu),
indium (In), gallium (Ga) and selenium (Se) as a light absorbing
layer is particularly attractive, because the CIGS solar cell is
excellent in sunlight conversion efficiency (hereinafter referred
to simply as "conversion efficiency") and is produced without the
use of silicon.
[0003] As shown in FIG. 8, the CIGS solar cell typically includes a
substrate 81, and a rear electrode layer 82, the CIGS film 83, a
buffer layer 84 and a transparent electrically-conductive film 85
provided in this order on the substrate 81.
[0004] An exemplary method for producing the CIGS film (light
absorbing layer) 83 of the CIGS solar cell is a so-called
three-step method which is capable of imparting the CIGS film with
a higher conversion efficiency. In this method, three steps are
performed after the rear electrode layer 82 is formed on a front
surface of the substrate 81. In the first step, In, Ga and Se are
vapor-deposited on a front surface of the rear electrode layer 82
to form an (In,Ga).sub.2Se.sub.3 film. In the second step, the
temperature of the substrate 81 is increased to 550.degree. C., and
Cu and Se are further vapor-deposited, whereby a Cu-rich CIGS film
intermediate product is formed. At this stage, two phases, i.e.,
liquid phase Cu.sub.(2-x)Se and solid phase CIGS, coexist in the
CIGS film intermediate product, whereby crystal grain size is
rapidly increased in the presence of Cu.sub.(2-x)Se. It is known
that Cu.sub.(2-x)Se has a lower resistance and, therefore,
adversely influences solar cell characteristics. In the third step,
therefore, In, Ga and Se are further vapor-deposited to reduce the
proportion of Cu.sub.(2-x)Se. Thus, the CIGS film 83 has a
composition slightly rich in Group III as a whole. The CIGS film 83
thus formed by the three-step method has greater crystal grain
diameters and yet has a thin film crystal structure having a
crystallographically higher quality (see, for example, PTL 1).
[0005] The CIGS film 83 formed in the aforementioned manner has a
V-shaped Ga/(In+Ga) ratio profile (so-called double-graded
structure) such that the Ga/(In+Ga) ratio is progressively reduced
along its thickness toward a predetermined thickness position 83a
(see FIG. 8) from a back surface of the CIGS film 83 (an interface
between the CIGS film 83 and the rear electrode layer 82) and is
progressively increased toward a front surface of the CIGS film 83
from the predetermined thickness position 83a as shown in FIG. 9.
The CIGS solar cell (see FIG. 8) employing the CIGS film 83 as the
light absorbing layer has a higher conversion efficiency. The
Ga/(In+Ga) ratio is herein defined as follows:
(A) the ratio of a gallium (Ga) atomic number concentration to the
sum of an indium (In) atomic number concentration and the gallium
(Ga) atomic number concentration.
RELATED ART DOCUMENT
Patent Document
[0006] PTL 1: JP-A-HEI10(1998)-513606
SUMMARY OF INVENTION
[0007] However, some CIGS solar cell having the aforementioned
construction still has a significantly lower conversion efficiency
with significant variation.
[0008] The inventors of the present invention conducted studies to
clarify the cause of the aforementioned problem. As a result, the
inventors found that the problem is attributable to oxidation of
the front surface of the CIGS film 83 (in contact with the buffer
layer 84). That is, the CIGS film 83 having the double-graded
structure has a Ga/(In+Ga) ratio progressively increased toward the
front surface of the CIGS film 83 from the predetermined thickness
position 83a, so that Ga is present in a higher proportion in the
front surface. Ga is more susceptible to oxidation than In.
Therefore, oxidation of Ga is more liable to proceed when the front
surface of the CIGS film 83 is exposed to air (oxygen) for a longer
period of time. Where the CIGS solar cell is produced by forming
the buffer layer 84 and the transparent electrically-conductive
film 85 on the front surface of the CIGS film 83 suffering from the
oxidation of Ga, the CIGS solar cell has a significantly reduced
conversion efficiency with significant variation.
[0009] In view of the foregoing, it is an object of the present
invention to provide a CIGS film production method capable of
suppressing the oxidation of the front surface, and a CIGS solar
cell production method using the CIGS film production method for
producing a CIGS solar cell substantially free from the reduction
and the variation in conversion efficiency.
[0010] According to a first aspect of the present invention to
achieve the aforementioned object, there is provided a production
method for a CIGS film to be used as a light absorbing layer for a
CIGS solar cell, the method including the steps of: forming a first
region having a Ga/(In+Ga) ratio progressively reduced as the
thickness of the first region increases to a predetermined
thickness from a back surface of the CIGS film; forming a second
region on the first region, the second region having a Ga/(In+Ga)
ratio progressively increased toward a front surface of the CIGS
film; and forming a third region on the second region by
vapor-depositing selenium (Se) and indium (In), the third region
having a Ga/(In+Ga) ratio progressively reduced toward the front
surface of the CIGS film; the Ga/(In+Ga) ratios each being defined
as follows:
(A) the ratio of a gallium (Ga) atomic number concentration to the
sum of an indium (In) atomic number concentration and the gallium
(Ga) atomic number concentration.
[0011] According to a second aspect of the present invention, there
is provided a CIGS solar cell production method, which includes the
step of forming a rear electrode, a light absorbing layer, a buffer
layer and a transparent electrically-conductive film in this order
on a substrate; wherein the light absorbing layer is a CIGS film
formed by the aforementioned CIGS film production method; wherein
the CIGS film has a back surface located adjacent to the rear
electrode.
[0012] In the present invention, the atomic number concentrations
may be each measured, for example, by means of an energy dispersive
fluorescent X-ray analyzer (EX-250 available from Horiba
Corporation) or a D-SIMS (dynamic SIMS) evaluation apparatus
(available from Ulvac-Phi, Inc.)
[0013] In the inventive CIGS film production method, selenium (Se)
and indium (In) are vapor-deposited on the front surface of the
to-be-formed CIGS film to form the third region. That is, the
formation of the third region does not involve formation of a
gallium (Ga)-containing film so that, in the third region, the
Ga/(In+Ga) ratio is progressively reduced toward the front surface
of the CIGS film from the second region provided below the third
region. Therefore, oxidation-susceptible Ga is present in a lower
proportion in the front surface. Thus, even if the front surface is
exposed to air (oxygen) for a longer period of time, the oxidation
can be suppressed. Where the CIGS solar cell is produced by
employing the CIGS film as the light absorbing layer, the CIGS
solar cell is substantially free from the reduction and the
variation in conversion efficiency.
[0014] In the inventive CIGS solar cell production method, the
light absorbing layer is formed by the aforementioned inventive
CIGS film production method, and the back surface of the CIGS film
is located adjacent to the rear electrode layer. In the inventive
CIGS solar cell production method, the oxidation of the front
surface of the CIGS film is suppressed and, in this state, the
buffer layer is formed on the front surface of the CIGS film. Thus,
the CIGS solar cell can be produced in which the reduction and the
variation in conversion efficiency are effectively suppressed.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is a sectional view schematically illustrating a CIGS
solar cell produced by a CIGS solar cell production method
according to one embodiment of the present invention.
[0016] FIG. 2 is a graph schematically showing a variation in
Ga/(In+Ga) ratio observed along the thickness of a CIGS film formed
by a CIGS film production method according to the embodiment of the
present invention.
[0017] FIGS. 3A to 3D are schematic diagrams for explaining a
production method for the solar cell.
[0018] FIGS. 4A and 4B are schematic diagrams following FIGS. 3A to
3D for explaining the solar cell production method.
[0019] FIGS. 5A and 5B are schematic diagrams following FIGS. 4A
and 4B for explaining the solar cell production method.
[0020] FIGS. 6A and 6B are schematic diagrams following FIGS. 5A
and 5B for explaining the solar cell production method.
[0021] FIGS. 7A and 7B are schematic diagrams following FIGS. 6A
and 6B for explaining the solar cell production method.
[0022] FIG. 8 is a sectional view schematically illustrating a
conventional CIGS solar cell.
[0023] FIG. 9 is a graph schematically showing a variation in
Ga/(In+Ga) ratio observed along the thickness of a conventional
CIGS film.
DESCRIPTION OF EMBODIMENTS
[0024] Next, an embodiment of the present invention will be
described in detail based on the attached drawings.
[0025] FIG. 1 is a sectional view schematically illustrating a CIGS
solar cell produced by a CIGS solar cell production method
according to the embodiment of the present invention. The CIGS
solar cell according to this embodiment includes a substrate 1, and
a rear electrode layer 2, a CIGS film 3, a buffer layer 4 and a
transparent electrically-conductive film 5 provided in this order
on the substrate 1. The CIGS film 3 is produced by a CIGS film
production method according to the embodiment of the present
invention. As shown in FIG. 2, the Ga/(In+Ga) ratio is
progressively reduced along the thickness of the CIGS film 3 toward
a predetermined first thickness position 3a (see FIG. 1) in a first
region 31 extending from a back surface of the CIGS film 3 located
adjacent to the rear electrode layer 2 to the first thickness
position 3a. The Ga/(In+Ga) ratio is progressively increased along
the thickness of the CIGS film 3 (toward a front surface of the
CIGS film 3) in a second region 32 provided on the first region 31
and extending to a second thickness position 3b (see FIG. 1).
Further, the Ga/(In+Ga) ratio is progressively reduced along the
thickness of the CIGS film 3 (toward the front surface) in a third
region 33 provided on the second region 32 and extending to the
front surface. Thus, the third region 33 in which
oxidation-susceptible Ga is present in a lower proportion is
provided adjacent to the front surface of the CIGS film 3, so that
the CIGS film 3 is substantially free from the oxidation of the
front surface thereof. This is a major feature of the present
invention.
[0026] The CIGS solar cell may be produced by the following
production method.
[0027] First, the substrate 1 (see FIG. 3A) is prepared. The
substrate 1 serves as a support substrate, and is made of a
material capable of enduring a temperature of not lower than
520.degree. C. to withstand heating in the subsequent heating step.
Examples of the material include soda lime glass (SLG), stainless
steel and titanium. Particularly, ferrite SUS430 is preferred for
workability.
[0028] Then, as shown in FIG. 3A, the rear electrode layer 2 is
formed on a front surface of the substrate 1 by a sputtering method
or the like. Exemplary materials for the rear electrode layer 2
include molybdenum, tungsten, chromium and titanium. The rear
electrode layer 2 may have a single layer structure or a multilayer
structure. The rear electrode layer 2 preferably has a thickness of
100 nm to 1000 nm.
[0029] Subsequently, as shown in FIG. 3B, a gallium selenide film
31A is formed on a front surface of the rear electrode layer 2 by a
vapor deposition method, and then an indium selenide film 31B is
formed on a front surface of the gallium selenide film 31A by a
vapor deposition method in order to form the first region 31 of the
CIGS film 3 (see FIG. 1) on the front surface of the rear electrode
layer 2. In turn, as shown in FIG. 3C, a gallium selenide film 31A
is formed on a front surface of the indium selenide film 31B in the
same manner as described above by the vapor deposition, and then an
indium selenide film 31B is formed on a front surface of the
gallium selenide film 31A by the vapor deposition. These steps are
repeated to stack one or more stacks 310 (three stacks in FIG. 3D)
each including the gallium selenide film 31A provided on the lower
side and the indium selenide film 31B provided on the upper side as
shown in FIG. 3D.
[0030] At this time, the thickness ratio (Y/X) between the
thickness (Y) of the gallium selenide film 31A and the thickness
(X) of the indium selenide film 31B in each of the stacks 310 is
set so that, in the first region 31 being formed, the Ga/(In+Ga)
ratio is progressively reduced as the thickness of the first region
31 is increased as described above. In this embodiment, the
thickness (X) of the indium selenide film 31B is set constant, and
the thickness (Y) of the gallium selenide film 31A is reduced as
the stacking is repeated. Thus, the thickness ratio (Y/X) is
reduced as the stacking is repeated. The thickness is controlled,
for example, by controlling the temperature of an evaporation
source such as Ga (i.e., the thickness is increased by increasing
the temperature, and reduced by reducing the temperature) or by
controlling the diameter of the opening of the evaporation source
(i.e., the thickness is increased by increasing the opening
diameter, and reduced by reducing the opening diameter). In order
to optimize the Ga/(In+Ga) ratio in the first region 31 being
formed, the first stack 310 preferably has a thickness ratio (Y/X)
of 0.5 to 1.3, and the last stack 310 preferably has a thickness
ratio (Y/X) of 0.2 to 0.5 which is smaller than the thickness ratio
(Y/X) of the first stack 310.
[0031] After the last stack 310 is formed, as shown in FIG. 4A, Cu
and Se are vapor-deposited on a front surface of the indium
selenide film 31B provided on the upper side of the last stack 310
for crystal growth, whereby a vapor deposition (copper selenide)
layer 31C is formed. Thus, a layered structure .alpha. including
the stacks 310 and the single vapor-deposition layer 31C is
produced. In the step of producing the layered structure .alpha.,
the substrate 1 is preferably maintained at a retention temperature
of 251.degree. C. to 400.degree. C., more preferably 290.degree. C.
to 360.degree. C. If the substrate retention temperature is higher
than 400.degree. C., diffusion is liable to occur in the vapor
deposition layer 31C during the formation of the vapor deposition
layer 31C, thereby preventing uniform crystal growth in the
subsequent step. In addition, Se is liable to re-evaporate from the
stacks 310, thereby impairing the crystal quality.
[0032] Thereafter, as shown in FIG. 4B, the layered structure
.alpha. is heated to not lower than 520.degree. C. for crystal
growth, whereby the first region 31 of the CIGS film 3 is
completed. That is, the heating liquefies the vapor deposition
(copper selenide) layer 31C and homogenously diffuses Cu throughout
the layered structure .alpha. to cause the crystal growth.
Therefore, the first region 31 thus formed is thicker than the
layered structure .alpha.. In the first region 31 thus formed, the
Ga/(In+Ga) ratio is progressively reduced along the thickness of
the first region 31 from the back surface (see FIG. 2).
[0033] Then, as shown in FIG. 5A, the temperature is maintained at
not lower than 520.degree. C., and one or more stacks 320 (two
stacks in FIG. 5A) each including a gallium selenide film 32A
provided on a lower side and an indium selenide film 32B provided
on an upper side are stacked on the first region 31 in the same
manner as described above (see FIGS. 3B to 3D) to form the second
region 32 of the CIGS film 3 on a front surface of the first region
31.
[0034] At this time, the thickness ratio (Y/X) between the
thickness (Y) of the gallium selenide film 32A and the thickness
(X) of the indium selenide film 32B in each of the stacks 320 is
set so that, in the second region 32 being formed, the Ga/(In+Ga)
ratio is progressively increased toward the front surface from the
first region 31 as described above. In this embodiment, the
thickness (X) of the indium selenide film 32B is set constant, and
the thickness (Y) of the gallium selenide film 32A is increased as
the stacking is repeated. Thus, the thickness ratio (Y/X) is
increased as the stacking is repeated. In order to optimize the
Ga/(In+Ga) ratio in the second region 32 being formed, the first
stack 320 preferably has a thickness ratio (Y/X) of 0.2 to 0.5, and
the last stack 320 preferably has a thickness ratio (Y/X) of 0.5 to
1.3 which is greater than the thickness ratio (Y/X) of the first
stack 320.
[0035] In this step, as described above, the formation of the
gallium selenide film 32A and the indium selenide film 32B is
achieved by the vapor deposition with the temperature maintained at
not lower than 520.degree. C., so that the crystal growth occurs in
these films 32A, 32B upon the formation of the films 32A, 32B by
the vapor deposition. Therefore, the second region 32 thus formed
has a thickness greater than the total thickness of the films 32A,
32B. In this manner, the second region 32 is formed as shown in
FIG. 5B. In the second region 32, the Ga/(In+Ga) ratio is
progressively increased toward the front surface of the CIGS film 3
from the first region 31 (see FIG. 2). The Ga/(In+Ga) ratio
preferably has a peak value of 0.3 to 0.6 so that the conversion
efficiency of the produced CIGS solar cell can be maintained at a
higher level with smaller variation.
[0036] Subsequently, as shown in FIG. 6A, the temperature is
maintained at not lower than 520.degree. C., and an indium selenide
film 33B is formed on the second region 32 in the same manner as
described above by the vapor deposition to form the third region 33
of the CIGS film 3 (see FIG. 1) on the front surface of the second
region 32. Since the formation of the indium selenide film 33B is
achieved by the vapor deposition with the temperature maintained at
not lower than 520.degree. C. in this step, as described above, the
crystal growth occurs in the indium selenide film 33B upon the
formation of the indium selenide film 33B by the vapor deposition.
Therefore, the third region 33 thus formed has a greater thickness
than the indium selenide film 33B. In this manner, the formation of
the CIGS film 3 including the first to third regions 31, 32 and 33
is completed as shown in FIG. 6B by the formation of the third
region 33.
[0037] Since the formation of the third region 33 does not involve
the formation of the gallium-containing film, the Ga/(In+Ga) ratio
is progressively reduced toward the front surface of the CIGS film
3 from the second region 32 (see FIG. 2). The reduction in
Ga/(In+Ga) ratio is preferably 0.02 to 0.3 in order to maintain the
conversion efficiency of the produced CIGS solar cell at a higher
level with smaller variation and to suppress the oxidation in the
front surface of the CIGS film 3.
[0038] The third region 33 preferably has a thickness of 30 to 200
nm so as to properly suppress the oxidation in the front surface
while suppressing the reduction and the variation in conversion
efficiency with proper balance.
[0039] The composition ratio of Cu, In and Ga of the CIGS film 3
preferably satisfies an expression of 0.70<Cu/(In+Ga)<0.95
(molar ratio). In this case, Cu.sub.(2-x)Se is prevented from being
excessively incorporated into the CIGS film 3. In addition, the
CIGS film 3 is slightly Cu-deficient as a whole. The ratio of Ga
and In which are the same group elements is preferably
0.10<Ga/(In+Ga)<0.40.
[0040] The CIGS film 3 preferably has a thickness of 1.0 to 3.0
.mu.m, more preferably 1.5 to 2.5 .mu.m. If the thickness is
excessively small, the CIGS film 3 to be used as the light
absorbing layer has a reduced light absorption amount, so that the
resulting device is liable to have a poorer performance. If the
thickness is excessively great, on the other hand, a longer period
is required for the formation of the film, resulting in lower
productivity.
[0041] Then, as shown in FIG. 7A, the buffer layer 4 is formed on
the front surface of the CIGS film 3. The buffer layer 4 may have a
single layer structure such as of ZnMgO or Zn(O,S), or may have a
multilayer structure including a CdS layer and a ZnO layer. These
layers may be each formed by a proper method. For example, the CdS
layer may be formed by a chemical bath deposition method, and the
ZnO layer may be formed by a sputtering method. The buffer layer 4
is preferably made of a higher-resistance n-type semiconductor so
as to form a pn junction with the CIGS film 3. The buffer layer 4
preferably has a thickness of 30 to 200 nm whether it has the
single layer structure or the multilayer structure. Where the
buffer layer 4 includes plural types of layers stacked one on
another, the pn junction can be more advantageously formed with
respect to the CIGS film 3. If the pn junction can be properly
formed, the buffer layer 4 is not necessarily required to have the
multilayer structure.
[0042] Subsequently, as shown in FIG. 7B, the transparent
electrically-conductive film 5 is formed on the front surface of
the buffer layer 4 by a sputtering method or the like. Exemplary
materials for the transparent electrically-conductive film 5
include indium tin oxide (ITO), indium zinc oxide (IZO) and
aluminum zinc oxide (Al:ZnO). The transparent
electrically-conductive film 5 preferably has a thickness of 100 to
300 nm.
[0043] Thus, the CIGS solar cell is produced in which the rear
electrode layer 2, the CIGS film 3, the buffer layer 4 and the
transparent electrically-conductive film 5 are stacked in this
order on the substrate 1.
[0044] In the aforementioned CIGS solar cell production method, the
third region 33 containing oxidation-susceptible Ga in a lower
proportion is formed adjacent to the front surface of the CIGS film
3. Therefore, the CIGS film 3 is substantially free from the
oxidation in the front surface thereof. Further, the CIGS solar
cell produced by employing the CIGS film 3 effectively suppresses
the reduction and the variation in conversion efficiency.
[0045] As described above, the third region 33 containing
oxidation-susceptible Ga in a lower proportion is provided adjacent
to the front surface of the CIGS film 3. Even if a longer period is
required before the formation of the buffer layer 4 on the front
surface of the CIGS film 3 after the formation of the third region
33 (after the formation of the CIGS film 3) and, therefore, the
front surface of the third region (the front surface of the CIGS
film 3) is exposed to air (oxygen) for a longer period of time, the
oxidation of the front surface can be suppressed. That is, even if
a period between the formation of the third region 33 (the
formation of the CIGS film 3) and the formation of the buffer layer
4 is longer, the reduction and the variation in the conversion
efficiency of the produced CIGS solar cell are not significantly
influenced. This makes the CIGS solar cell production method more
flexible to optimize the production control.
[0046] Where the CIGS film 3 is formed by stacking the stacks 310,
320 as in this embodiment, the formation of the CIGS film 3 can be
achieved with higher reproducibility of the Ga/(In+Ga) ratio of the
CIGS film 3. As a result, the CIGS solar cell can be stably
produced as having a higher conversion efficiency.
[0047] In the embodiment described above, the gallium selenide film
31A, 32A is provided on the lower side and the indium selenide film
31B, 32B is provided on the upper side in each of the stacks 310
provided one on another to form the first region 31 of the CIGS
film 3 and in each of the stacks 320 provided one on another to
form the second region 32 of the CIGS film 3, but these layers may
be stacked in a reverse order (the indium selenide film 31B, 32B
may be provided on the lower side, and the gallium selenide film
31A, 32A may be provided on the upper side).
[0048] In a CIGS film production method according to another
embodiment of the present invention, a new CIGS film is produced by
forming a CIGS film 83 (see FIG. 8) by the conventional three-step
method, and then forming an indium selenide film 33B (see FIG. 6A)
by the vapor deposition on a front surface of the previously formed
CIGS film 83 in the same manner as in the previous embodiment. The
new CIGS film thus produced also has a Ga/(In+Ga) ratio
progressively reduced toward the front surface thereof. The other
arrangement of the CIGS film production method is the same as in
the previous embodiment.
[0049] Another new CIGS film may be produced by forming a CIGS film
having a V-shaped Ga/(In+Ga) ratio profile (double-graded
structure) (see FIG. 9) by a conventional production method other
than the three-step method and then forming an indium selenide film
33B (see FIG. 6A) by the vapor deposition on a front surface of the
previously formed CIGS film in the same manner as in the previous
embodiment.
[0050] In the embodiments described above, the CIGS solar cell is
configured so that the rear electrode layer 2, the CIGS film 3, the
buffer layer 4 and the transparent electrically-conductive film 5
are stacked in this order in contact with each other on the
substrate 1 but, as required, other layers may be provided between
adjacent constituent layers otherwise provided in contact with each
other, on the back surface of the substrate 1 and/or on the front
surface of the transparent electrically-conductive film 5.
[0051] Next, inventive examples will be described in conjunction
with a conventional example. It should be understood that the
present invention be not limited to these inventive examples.
EXAMPLES
Example 1
Preparation of Substrate and Formation of Rear Electrode Layer
[0052] A CIGS solar cell was produced in the same manner as in the
aforementioned embodiment. More specifically, a substrate of soda
lime glass (30 mm.times.30 mm.times.0.55 mm (thickness)) was
prepared, and a rear electrode layer of molybdenum (having a
thickness of 500 nm) was formed on a front surface of the substrate
by a sputtering method.
[0053] <Formation of First Region>
[0054] Then, a gallium selenide film (having a thickness of 130 nm)
was formed on a front surface of the rear electrode layer by means
of a vapor deposition apparatus while the substrate was maintained
at 330.degree. C. Thereafter, an indium selenide film (having a
thickness of 330 nm) was formed on a front surface of the gallium
selenide film. Subsequently, Cu and Se were vapor-deposited on a
front surface of the indium selenide film, whereby a vapor
deposition layer of copper selenide (having a thickness of 1400 nm)
was formed. In this manner, a stack including the gallium selenide
film, the indium selenide film and the copper selenide layer (vapor
deposition layer) was formed. Thereafter, the resulting substrate
was heated to be maintained at a substrate retention temperature of
550.degree. C. for 5 minutes while a very small amount of Se vapor
was supplied to the stack. Thus, the stack experienced crystal
growth, whereby a first region was formed.
[0055] <Formation of Second Region>
[0056] Subsequently, an indium selenide film was formed on a front
surface of the first region in the same manner as described above
by maintaining the substrate at 550.degree. C. while supplying a
very small amount of Se gas. Thereafter, a gallium selenide film
was formed on a front surface of the indium selenide film. At this
time, the gallium selenide film had a thickness of 30 nm, and the
indium selenide film had a thickness of 80 nm when the substrate
temperature reached 330.degree. C.
[0057] <Formation of Third Region>
[0058] Then, a single indium selenide film (having a thickness of
10 nm) was formed on a front surface of the second region in the
same manner as described above through vapor deposition by
maintaining the substrate at 550.degree. C. while supplying a very
small amount of Se gas. Thus, a third region was formed. In the
formation of the third region, a gallium-containing film was not
formed. Therefore, the Ga/(In+Ga) ratio was progressively reduced
toward the front surface of the CIGS film from the second region.
Thus, a CIGS film (having a thickness of 2.0 .mu.m) including the
first to third regions was formed.
Example 2
[0059] A CIGS film was formed in substantially the same manner as
in Example 1, except that the second region was formed by
simultaneously vapor-depositing selenium, indium and gallium in
Example 1.
Example 3
[0060] A CIGS film was formed in substantially the same manner as
in Example 2, except that the first region was formed by
simultaneously vapor-depositing selenium, indium and gallium for a
vapor deposition period of 25 minutes with selenium, indium and
gallium vapor deposition sources being kept at temperatures of
180.degree. C., 850.degree. C. and 1000.degree. C., respectively,
in Example 2.
Example 4
[0061] A CIGS film was formed in substantially the same manner as
in Example 3, except that the second region was formed by forming
an indium selenide film and then forming a gallium selenide film on
a front surface of the indium selenide film in Example 3.
Example 5
[0062] A CIGS film was formed in substantially the same manner as
in Example 1, except that the indium selenide film was formed as
having a thickness of 25 nm by the vapor deposition to form the
third region in Example 1.
Conventional Example
[0063] A CIGS film was formed in substantially the same manner as
in Example 1, except that the conventional three-step method was
employed. More specifically, a rear electrode layer was formed on
the front surface of the substrate in the same manner as in Example
1. Then, In, Ga and Se were simultaneously vapor-deposited with the
substrate maintained at a substrate retention temperature of
350.degree. C., whereby a layer of In, Ga and Se was formed. While
the substrate was heated to be maintained at a substrate retention
temperature of 550.degree. C., Cu and Se were vapor-deposited on
the layer of In, Ga and Se, and allowed for crystal growth. Thus, a
CIGS film intermediate product was obtained. Further, In, Ga and Se
were simultaneously vapor-deposited on the CIGS film intermediate
product by maintaining the substrate at a substrate retention
temperature of 550.degree. C. while supplying a very small amount
of Se vapor to the CIGS film intermediate product. Thus, a CIGS
film (having a thickness of 2.0 .mu.m) was formed.
[0064] <Formation of Buffer Layer and Transparent Electrode
Layer>
[0065] For each of Examples 1 to 5 and Conventional Example, two
CIGS films were prepared. Within two hours after the formation of
one of the two CIGS films (within two hours during which one of the
CIGS films was exposed to air), a CdS layer (having a thickness of
50 nm) was formed on a front surface of the CIGS film by a chemical
bath deposition method, and then a ZnO layer (having a thickness of
70 nm) was formed on a front surface of the CdS layer by a
sputtering method. Thus, a buffer layer including the CdS layer and
the ZnO layer was formed. In turn, a transparent electrode layer of
ITO (having a thickness of 200 nm) was formed on a front surface of
the buffer layer by a sputtering method. Thus, a CIGS solar cell
was produced. The other CIGS film was exposed to air for 24 hours
after the formation thereof, and a buffer layer and a transparent
electrode layer were formed on a front surface of the CIGS film in
the same manner as described above. Thus, another CIGS solar cell
was produced.
[0066] [Measurement of Conversion Efficiency]
[0067] For each of Examples 1 to 5 and Conventional Example, the
conversion efficiency of the CIGS solar cell formed with the buffer
layer within two hours after the formation of the CIGS film and the
conversion efficiency of the CIGS solar cell formed with the buffer
layer after a lapse of 24 hours from the formation of the CIGS film
were each measured by applying artificial sunlight (AM1.5) to an
area over the front surface of the CIGS solar cell by means of a
solar simulator (CELL TESTER YSS150 available from Yamashita Denso
Corporation). The results are shown below in Table 1.
TABLE-US-00001 TABLE 1 Conversion Example Conventional efficiency
(%) 1 2 3 4 5 Example Within 2 hours 15.7 15.8 15.8 15.9 15.7 13.8
After 24 hours 15.4 15.5 15.5 15.6 15.4 12.8
[0068] The results shown in Table 1 indicate that the CIGS solar
cells of Examples 1 to 5 each had a higher conversion efficiency
than the CIGS solar cell of Conventional Example and, even where
the CIGS films were exposed to air for a longer period time, the
conversion efficiencies were not significantly reduced in Examples
1 to 5 as compared with Conventional Example. This is because the
front surfaces of the CIGS films of Examples 1 to 5 were less
susceptible to oxidation than the front surface of the CIGS film of
Conventional Example even when being exposed to air. This is
attributable to the fact that the CIGS films of Examples 1 to 5
each had the third region in which oxidation-susceptible Ga was
present in a lower proportion, while the CIGS film of Conventional
Example had no such region and hence had a higher Ga proportion in
the front surface.
[0069] While specific forms of the embodiment of the present
invention have been shown in the aforementioned inventive examples,
the inventive examples are merely illustrative of the invention but
not limitative of the invention. It is contemplated that various
modifications apparent to those skilled in the art could be made
within the scope of the invention.
[0070] The inventive CIGS film production method is used for
producing a CIGS film substantially free from the oxidation of the
front surface of the CIGS film, and the inventive CIGS solar cell
production method is used for producing a CIGS solar cell
substantially free from the reduction and the variation in
conversion efficiency.
* * * * *